Polymer based strain gauges have been patented to work as piezoresistive sensors or transducers in the last few decades, as for example in U.S. Pat. No. 5,505,093A, GB2141548A, U.S. Pat. Nos. 8,661,917B2, 4,765,930A, 6,951,143B1 and US20140260653A1. In general, these documents reveal materials for the development of force sensors or devices comprising conductive polymers and silicone rubbers as matrices. The materials used as electrical conductors are based on carbon nano- and microstructures, conductive polymers or inorganic fillers.
Piezoresistive behaviour can be described as a mechanical stimulus that induces in the sensor a change in electrical resistivity, as mentioned in document U.S. Pat. No. 2,951,817, which describes a polyvinyl chloride polymer matrix with manganese dioxide as filler, where the electrical resistance changes throughout a very wide range of values in response to very small deformations. Resistive deformable sensors touch screens for application in electronic devices are presented in the document US20100123686A. Piezoresistive pressure sensors chips that are exposed to the external pressure medium directly have been disclosed in U.S. Pat. No. 8,567,256B2, as well as pressure sensors for the measurement of compression and tension of materials in different applications, which are disclosed in WO 2007044307 A1.
Among the different used electrical conductive nanostructures, carbon nanotubes (CNTs) have received special attention in research and industry, as observed in documents U.S. Pat. Nos. 7,318,351 B2 A or 7,553,681 B2. By increasing fillers content on the polymeric matrix until a critical amount it is observed a large increase in the electrical conductivity of the inks. After this region, the conductivity tends to stabilize. This phenomenon is referred as the electrical percolation threshold, and this parameter is dependent on the polymer matrix, nanostructures and preparation methods. The percolation threshold plays an important role on the piezoresistive response in these kind of composites, as the ink formulation should work preferentially around that concentration.
The majority of the referenced documents in this area, such as U.S. Pat. No. 7,318,351B2, and WO2007044307A1 reveal the development of piezoresistive pressure sensors for micro and macro deformations. In the WO2007044307A1 document the nitrogen implantation in different polymers and conductive polymer materials using polyaniline (PANI) as piezoresistive thin films, which shows interesting piezoresistive properties and can be implemented directly in whole substrate. These are presented as an alternative to metal film and silicon piezoresistive sensors. The GB2141548A discloses a transducer incorporating an electrical resistance strain gauge element in the form of a conductive polymer comprising a dispersion of electrically conductive or resistive particles in an electrically insulating organic polymer. The particles comprise a dispersion of electrically conductive carbon in an organic polymer such as epoxy, alkyd, polyester, acrylic or silicon materials or copolymers thereof. The U.S. Pat. No. 5,505,093A discloses that certain thermally stable homogeneously conductive films unexpectedly exhibit effective piezoresistivity, when employed in the measurement of strain, for the measurement of large flexural deformations. These transducers can comprise either one of two alternative types of polymers, i.e., solvent soluble polyaniline conducting polymers, or ion-implanted polymers. The polymers are patternable by photolithography. The U.S. Pat. No. 4,708,019A discloses that certain piezoresistive blends of doped acetylene polymers and elastomers can be employed in the measurement of strain due to stress applied to a sample. Pressure transducers, such as strain gauges, can be fabricated employing such blends. These transducers contain a polymeric element capable of deformation in response to stress applied to the sample. The polymeric element comprises a piezoresistive blend of an acetylene polymer doped to make it electrically conductive.
A related document U.S. Pat. No. 9,032,804, deals with large-area extensible pressure sensor for textiles surfaces, comprising a support or substrate on which a conductive and piezoresistive paste or ink is printed. This paste or ink is composed by conductive or graphite materials on a polymeric matrix of elastic deformable material such as silicone.
These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.